To increase the capacity of communications systems, wavelength division multiplexing systems have been developed. Such systems transmit signals on a plurality of wavelengths. At the receiver, or at intermediate points for channel monitoring, it is frequently desirable or necessary to select a particular wavelength signal from a group of several wavelengths. The strength of each channel can then be measured, channels can be selectively amplified, or problems such as missing channels or excessive signal to noise (S/N) can be detected.
Wavelength calibration is typically performed by comparing a reference signal to the transmitted or received signal. Reference signals may include a stable wavelength laser source or coherent beat or other means. As disclosed in U.S. Pat. No. 5,283,845 issued to JDS Fitel Inc. in 1994, a known broadband source is provided through a wavelength monitor and control circuit to adjust a tunable etalon filter spacing. However these referencing techniques require more complicated input transmission devices which generate known frequencies and more complicated detecting circuits for performing wavelength comparisons. To monitor a plurality of channels through a single tunable filter assembly providing a known wavelength source to tune the filter for each channel would be difficult, cumbersome and expensive.
In a prior art system disclosed in U.S. Pat. No. 5,646,762 issued to Lucent Technologies, a reference signal is provided by a separate source into a tunable sampling filter. Example reference sources include a semiconductor laser, a fiber ring laser or a fiber grating laser. However, the filter calibration, and the resulting sampled channel information are only as accurate as the known wavelength of the reference signal. These stable wavelength lasers are expensive, and subject to a degree of error.
It is necessary in large multiple channel systems to be able to monitor the signals precisely. It is desired to provide a simplified referencing system for identifying multiple channels without impeding signal transmission. It is also important to know the signal to channel noise ratio to properly assess signal quality. This capability does not exist in the prior art for monitoring multiple channel systems.